1. Field of the Invention
The present invention relates to an amplifying circuit, and in particular to an amplifying circuit of class AB type, comprising a complementary pair of output transistors.
2. Discussion of the Related Art
FIG. 1 illustrates an amplifying circuit 100 of class AB type, and reproduces a part of an amplifying circuit described in the publication “A CMOS Low-Distortion Fully Differential Power Amplifier with Double Nested Miller Compensation”, Sergio Pernici et al., IEEE.
Amplifier 100 comprises an input node 102, and an output node 104. The output node 104 is coupled to a complementary pair of output power transistors 106 and 108 forming an output stage of class AB type. Transistors 106 and 108 are coupled in series between ground and positive supply rails. Transistor 106 is a P-channel MOS transistor while transistor 108 is an N-channel MOS transistor. The gate node of transistor 108 is coupled to the input node 102. The input node 102 is also coupled to a control stage for controlling the PMOS transistor 106. In particular, input node 102 is coupled to the gate node of an N-channel MOS transistor 110. Transistor 110 has its source terminal coupled to the ground voltage rail and its drain terminal coupled to a node 111 via an N-channel MOS transistor 112. Node 111 is further coupled to the positive supply rail via a P-channel MOS transistor 114, having its gate node coupled to node 111. PMOS 114 forms a current mirror configuration with a P-channel MOS transistor 116, which also has its gate node coupled to node 111. PMOS 116 is coupled between the positive supply rail and a node 117, which is coupled to the gate node of P-channel MOS transistor 106. Node 117 is also coupled to the ground supply rail via further N-channel MOS transistor 113 and a current source 118. Transistors 112 and 113 have their gate nodes connected to a fixed voltage VB.
In operation, the output transistors 106 and 108 of the class AB type allow of the amplifier to drive an output load from either the positive or ground supply rail. It has been found that at relatively high frequencies of the input signal and at relatively low loads, for example of around 2 k Ohms or less, the signal at the output node 104 of the amplifier 100 tends to be distorted. There is thus a need for an improved amplifying circuit that does not suffer such distortions.